This invention relates to a method and an apparatus for measuring and monitoring the partial density of metal and acid in pickling baths.
When metal surfaces are pickled in order to remove deposits, usually oxide deposits, such as roll scale, hammer scale, corrosion films and the like, or to roughen them for special further processing purposes, or to clean the metal surfaces, inorganic and organic acids are used.
Chemical descaling after thermal deformation, for instance with a semi-finished product made of iron and iron alloys, is performed in mineral acids such as sulfuric acid, mixtures of nitric acid and hydrofluoric acid, or phosphoric acid.
The predominant reaction products of the pickling process are ferrous ions, as cations of the ferrous salt of the applicable pickling acid present in solution, and water, until the critical free iron surface is attained; upon further reduction of the mixed metal and metal oxide potential, atomic hydrogen occurs as well, which recombines into molecular hydrogen at lattice vacancies and forms gas bubbles.
When the solution equilibrium is exceeded, iron salts crystallize out, in various hydration forms that depend on the temperature and concentration.
For the design and operation of a pickling line the pickling speed is the essential factor; it is not only affected by the tendency of scale formation, but above all is a function of the acid concentration and the iron content, which increases with the dissolution of the scale. Other important factors are the temperature of the pickling solution and the movement of the material being pickled; other factors that affect the pickling time are the addition of an inhibitor and the presence of metallic and nonmetallic contaminants and impurities in the pickling solution.
The salt content in the various pickling acids has a variable effect on the pickling speed. With sulfuric acid, for example, an increasing content of ferrous sulfate reduces the pickling speed, and the ferrous ions have an inhibiting effect on the iron attack; on the other hand, with hydrochloric acid the pickling time decreases as the ferrous chloride content rises, until just below the limit of saturation, and the iron attack remains unretarded.
Modern pickling methods are coupled with regeneration systems for processing the used pickling solution. In pickling with sufuric acid, for example, the ferrous sulfate that forms must be continuously removed from the pickling process and the quantity consumed must be replenished with fresh sulfuric acid; in the case of hydrochloric acid, the used pickling solution can be regenerated virtually completely, that is, it is unnecessary to replenish it with fresh acid.
If the drop in the acid content is signalled in good time, a prolongation of the pickling time can be avoided by increasing the delivery of fresh acid. Conversely, the acid consumption can be reduced by avoiding an overly high acid content in the pickling solution. By monitoring the acid and iron content and accurately adjusting them, the outcome of pickling can be made more uniform for the same material to be pickled, and the capacity of the regeneration system can thus be more uniformly exploited.
The monitoring of industrial pickling baths is done predominantly by manual titration, for example by the titration of the free acid with caustic soda (NaOH) and the titration of the ferrous content with potassium permanganate (KMnO.sub.4) or potassium dichromate (K.sub.2 Cr.sub.2 O.sub.7). Here, Fe.sup.2+ is oxidized into Fe.sup.3+ ; this means that existing Fe.sup.3+ in industrial pickling acid is not detected, in this method.
The prevailing loyalty to this simple manual method in the industry is explained by the fact that in pickling lines having a fixed, monitorable pickling program, periodic monitoring at intervals of 2 or 4 hours is normally sufficient, so that the use of automatic measuring methods is not yet considered to be absolutely necessary.
The situation is different in pickling lines in which the programs change, at short time intervals, betweeen material that is easy to pickle to material that is difficult to pickle. The pickling temperature, acid concentration and duration of pickling must be adapted continuously to the variable capacity for pickling of the material to be pickled, and the iron contents vary accordingly. The continuous changes necessitate monitoring of the pickling process at very much shorter time intervals; although certain relationships can be demonstrated retroactively by analysis, as a rule this is too late for intervention in the pickling process if an adaptation to the pickling program is to be made.
Various attempts have been made to replace manual titration with modern process titration and thus to drastically shorten the rate of monitoring. However, it has been found that the equipment used for this purpose, although it is successfully used for monitoring water or in the foodstuffs industry for instance, does not function reliably enough under the heavy-duty conditions of industrial metallurgy. The burettes very quickly become contaminated, so that the required measurement accuracy becomes questionable. Hence, frequent cleaning, which is time-consuming, is required.
Process titrators are also being used in combination with photometric measuring methods, the latter used for determining the iron content. In photometric measurement, the ferric component can be ascertained indirectly, as a difference between the total iron (in solution), determined with thioglycol acid, and the ferrous component, for instance determined with ortho-phenanthroline.
Because of their sensitivity to contaminants in the pickling solution, photometric measuring methods are usable only under limited conditions. Industrial pickling acid having a fluctuating content of hydrated salts, colloidally precipitated silicates (SiO.sub.2 . aq), etc., contaminate the measurement cells. The gases and impurities forming during scale dissolution also have a perturbing effect. In this state, the pickling acid is not a pure solution but rather a suspension. To retain the suspended particles, filters disposed in the inlet side are used. These filters must be changed frequently. Testing, cleaning and recalibration must be performed repeatedly, making for a kind of operation which is very expensive for management and does not meet the required level of safety in industrial pickling baths.
The density and proportions of substances in acidic, aqueous ferrous salt solutions can be brought into a mathematical relationship sufficiently accurate for practical purposes; see J. Pearson and W. Bullough, J. Iron Steel Inst. 167 (1951), pp. 439-445; W. Fackert, Z. Stahl & Eisen [Iron and Steel Journal] 72 (1952), pp. 1196-1207; and G. Dunk and B. Meuthen, Z. Stahl & Eisen 82 (1962), pp. 1790-1796. The density of the solution is calculated from the concentrations of acid and iron. For one variable to be calculated, the other two must be known. The relationships are valid only for a particular temperature; the effect of temperature on the density is not taken into account.
The following efforts have been made to determine the acid and iron content by taking density measurements into account:
U.S. Pat. No. 2,927,871 discloses how such a mathematical relationship between the density, the specific conductivity and the contents of acid and iron in sulfuric acid pickling baths can be used for designing a continuous-function monitoring apparatus. This apparatus comprises a density measurement probe (operating according to the air bubble method) that is immersed in the pickling solution, and a conductivity measuring cell that is immersed in the pickling solution. Problems arise due to the short service life of the measuring probe and the falsification of the conductivity measurement values resulting from the deposition of oil onto the glass electrodes (when oiled bands are subsequently pickled, lubricating oil gets into the pickling acid). It has also been found that this measuring method cannot be used when pickling with hydrochloric acid.
Recent efforts toward further development of this type of measuring method and its widespread introduction into industrial practice have failed because the measurement of conductivity has proven to be too difficult. Essentially, there are three reasons for this:
Firstly, the conductivity is usable as a measurement variable only for dilute solutions. With an increasing content of ion-forming constituents, the forces of interaction increasingly inhibit the mobility of the ions, so that the conductivity does not increase further even though pickling acids must be classified as powerful electrolytes.
Secondly, the conductivity responds to all ionized charge carriers, which can increase in quantity in the pickling baths, depending on the pickling program. This includes the cations Fe.sup.2+, Fe.sup.3+, Mn.sup.2+, Al.sup.3+, Cr.sup.3+ and the hydronium ion H.sub.3 O.sup.+, as well as the anions Cl.sup.-, SO.sub.4.sup.2-, PO.sub.4.sup.H3-. The conductivity is the product of elementary charge, the valence of the particular charge carrier, and the mobility and the number of the particles of the particular charge carrier. The more diverse the types of charge carriers, and the greater their number, the more complex are the electrochemical processes. No reliable information is available as to the mobility of the particles in concentrated solutions.
Finally, the production of hydrogen associated with the dissolution of scale is also a problem. This depends not only on the composition, thickness and properties of the scale film, but also on the inhibitor content; a measurement variable such as conductivity, which is greatly affected by the kinetics of this process, is understandably unsuitable for monitoring of the acid and iron contents in industrial pickling acids.
Japanese Patent 56 136 982 discloses a method for regulating constant concentrations of the acid content in pickling containers by metered replenishment of fresh acid or regenerate. The acid that is added is bound at a stoichiometric ratio by the iron present in the pickling bath. There is a linear relationship between the content of iron ions and the excess acid. If the acid content of the fresh acid supplied is known, then this relationship can easily be ascertained by a series of tests with graduated iron contents. The function thus obtained can now be used in one of the relationships, known from the professional literature, between density and acid and iron content, so that a mathematical relationship between density and acid content is obtained. This is supplemented with a temperature correction of the density.
With the aid of the relationship discovered in this way, the content of free acid in the pickling bath can be calculated from the density and temperature measured there, if the acid content of the incoming fresh acid is known. The calculation method is designed such that the determination of the iron content can be omitted. The result is used to regulate the supply of acid, the goal being to keep the content of free acid in the pickling container as uniform as possible.
The method has the disadvantage, however, that only the last pickling container supplied directly with fresh acid or regenerate can be monitored directly. As is well known, the content of acid and iron varies from container to container in the direction of band travel in a clearly graduated manner: While in sulfuric acid pickling, for example, acid contents of between 200 and 280 g/l and iron contents of between 60 and 100 g/l are found in the first container, the acid content in the final container ranges between 250 and 350 g/l, with iron contents between 20 and 60 g/l. The contents already fluctuate considerably in the first containers, as a function of the pickling program and the throughput; it is also difficult to check the change in the ratios in the first container resulting from a change in the supply of fresh acid in the final container. The temperature drop from the last monitored container to the first container into which the band runs also makes the control of the pickling process difficult.
In sulfuric acid pickling solutions, for example, which in addition to regenerate also require fresh acid for replenishment of used acid, or which in other words must be supplied from two sources at the same time, it is difficult to calculate beforehand how much acid must be replenished; among other factors, the influence of the heat of reaction must be taken into account. The ratios become even harder to check, if water is replenished as well.
It has been found that a measuring method that omits the checking of the iron content and furthermore monitors only the container coupled with the supply of fresh acid is inadequate to control the pickling process, in pickling lines in which the program changes frequently.
This substantial disadvantage of the previously known measuring method can be avoided only if it is possible to find a method that makes it possible to ascertain not only the acid contents but also the iron contents, and as much as possible in all containers regardless of the replenishment of acid. In that case, the precondition that the acid content of the fresh acid supplied must be constant and known is eliminated.
Japanese Patent 56 136 982 provides no information as to the type of density measurement, so that it does not teach whether the aforementioned disadvantages of density measurement found in U.S. Pat. No. 2,927,871 can be overcome.